As a method for reducing and purifying NOx in exhaust gas from an internal-combustion engine such as a diesel engine, there is suggested a method that utilizes a catalytic converter of a selective catalytic reduction system (SCR system) in which urea water is applied. The urea SCR system is a system that uses urea water as a reducing agent to mix the agent in an exhaust gas, thereby purifying the exhaust gas with an SCR catalytic converter. In the system, urea changes into ammonia in the exhaust gas, and NOx in the exhaust gas is linked with the ammonia in the SCR catalytic converter to be decomposed into water and nitrogen, and, therefore, the system is considered to be a promising technique for making exhaust gas clean.
Further, standardization about purity of an aqueous urea solution for use in a urea SCR system is under study. For example, German Institute for Standardization/Deutsche Industrie Normen (DIN) 70070 prescribes an aqueous urea solution for an in-vehicle SCR catalytic converter for diesel cars. In the standard, a 32.5% aqueous urea solution is prescribed. Meanwhile, in the present invention, hereinafter, an aqueous urea solution with high purity suitable as a reducing agent for an SCR application and the like is called a “high-purity aqueous urea solution.”
As a method for producing a high-purity aqueous urea solution, there is known a method for dissolving commercially available urea for fertilizer in purified water to form an aqueous solution. The production method is a comparatively simple method. However, generally, commercially available urea contains aldehyde, an anticaking additive, a coating agent and the like. These aldehyde, anticaking additive, coating agent and the like correspond to impurities for an aqueous urea solution for SCR. Therefore, in a conventional method for producing a high-purity aqueous urea solution, it is necessary to remove impurities from commercially available urea for fertilizer. Accordingly, the conventional method for producing a high-purity aqueous urea solution is not an effective method. Further, when the production method is to be utilized, it is necessary to pay attention also to acquisition of a raw material.
Consequently, it is tried to obtain a high-purity aqueous urea solution in a stage prior to production of urea corresponding to a “finished product” such as urea for fertilizer, that is, in a urea production process. For example, in PTL 1, there is suggested a method for partially separating an aqueous urea solution (UREA-COMPRISING AQUEOUS STREAM) produced in a urea production process, and diluting the aqueous urea solution with water to produce an aqueous urea solution of 30 to 35% by weight.